1. Technical Field
The present invention relates to load cells having one or more gaged beams as the load sensing element and more particularly to such load cells which are compensated for displacement of the load and to methods of so compensating such load cells.
2. Background Art
In weighing mechanisms employing a load cell with one or more gaged beams, an item to be weighed is usually placed upon a platter or platform supported by one end of the beam or beams. It is well known that errors may arise if the weight is placed "off center", or other than in the central position it occupied during calibration of the weighing mechanism. Off-center loading may be longitudinal, along the central longitudinal axis of an elongated beam load cell, or lateral, along an axis perpendicular to the longitudinal axis, or both.
If the beams and other elements of a load cell were uniform and completely symmetrical, it would be theoretically possible for the weight to be placed at different positions on a platter supported by the load cell without affecting the response of the load cell. In practice, however, errors arise from placement of the load at different positions on the platter. The errors arise from slight variations in many factors. These include: (1) small variations in the positions of the strain gages on the beam(s); (2) variations in the bonding of the strain gages to the beam(s); (3) strain gage geometry variations; (4) strain gage resistance variations; (5) strain gage factor variations; (6) variations in load cell geometry; (7) variation in the thickness of any reduced sections of the beam(s); (8) variation in the location and/or geometry of any reduced sections; (9) variations in the compensation of the load cell beam(s); (10) variation in the strain in the load cell due to machining and other factors; and (11) variations in the lead in the lead wires to the connecting circuitry. Various ways have been suggested for compensating for the effects of off-center loading.
U.S. Pat. No. 3,576,128 to Lockery discloses a dual beam load cell compensated only for longitudinal displacement of the load by connecting resistors to the strain gages of the load cell. U.S. Pat. No. 4,128,001 to Marks discloses a dual beam load cell compensated for lateral and longitudinal displacement of the load by removing material from certain locations on the beams. U.S. application of Griffen, Ser. No. 272,928, filed June 12, 1981, now U.S. Pat. No. 4,380,175 discloses an improved arrangement for compensating for both longitudinal and lateral displacement for a dual beam load cell.
U.S. Pat. No. 4,282,748 to Lockery et al relates to a single beam load cell wherein the radius of a fillet between the load section and the mounting section is adjusted so that the load cell becomes substantially insensitive to inboard-outboard loading. Co-pending U.S. application Ser. No. 358,195 of Griffen and Behrman, now U.S. Pat. No. 4,453,609 discloses an improved single beam load cell compensated for longitudinal and lateral displacement of the load. The compensation may be accomplished by honing in reduced areas of the beam or by mounting the strain gages at angles to the longitudinal axis of the beam and connecting appropriate resistor networks to the strain gages.
U.S. Pat. No. 4,107,985 to Sommer discloses a single beam load cell between a pair of parallel flexure members. Material is removed from the beam by honing or filing between longitudinally spaced strain gages thereon to reduce the effects of off-center loading.